Reuse shared logic between Lockup and Flow through a new "SablierBase" interface/abstract

[PaulRBerg]

The following functionality is duplicated across the Lockup and the Flow repositories:

• aggregateAmount
• collectFees
• nextStreamId
• recover
• setGasTokenAddress
• updateMetadata

Why not put it into a new shared abstract in this repo, @sablier-labs/evm?

And then, the interfaces for ILockupNFTDescriptor and IFlowNFTDescriptor could be merged into a single interface, again part of this repository.

Note: there's no need to rush this refactor into the current release if there's no time for it.

[smol-ninja]

We can move implementations that we are 100% sure won't change over time between Lockup and Flow. But I am not convinced if we should move variables as well.

• updateMetadata: its inherited through IERC4906. So, the implementation is already outsourced.
• aggregateBalance and nextStreamId : It would be a bit of a challenge to locate these variables for someone looking at the code for the first time. Therefore, I am not in favour of moving nextStreamId into the shared repo.
• recover and collectFees: We can. I don't think we would change their implementations between the two protocols. However, it would mean we will have to move aggregateBalance as well.
• setNativeToken: Sure if we don't change its implementation between the two.

Another argument against moving common code to this repo is that in case we decide to trim down Lockup contract (due to its size constraint) by removing unnecessary functions or refactor some of the functions in Lockup for efficiency reasons, we may revert this change.

Related:

• Remove `SablierFees` from this repo #4

[PaulRBerg]

OK these are good points against merging the code — let's wait for the full spec of the next release to be agreed upon, and then we can revisit this.